I know this may sound really dumb and obvious, but one of the first things I would do is swap my monitor and computer power cords. Leave them plugged into the same wall/power strip jack they’re currently in
I’ve done that for more than a few people and determined very quickly that the power cord was the culprit - it was shorted inside somewhere, or the plug holes were just to loose to maintain a continuous connection. Also, if the cord IS the problem, you won’t have to wait for your system to reboot (and the monitor going black suddenly would be proof positive the cord is bad )
Occasionally, once the power cords were switched, the problem never recurred, indicating the cord was most likely not plugged in all the way.
If the CPU still reboots, then swap where they’re plugged into the wall/power strip. If the CPU still reboots, you can safely eliminate everything from the wall to the CPU itself from being the culprit.
Before doing anymore hardware checking (since the rest of this will most likely cost $$ unless you have spares lying around), I would then do a reinstall or format/reinstall of XP to eliminate software issues.
Next thing I would do is get another Power Supply ($30-$40, new Power Supply back to the store you got it from and put the old one back in (no sense wasting the money ). From there, I would start swapping out the other parts where I could (get a cheap network and/or sound card, and disable the onboard cards in your system’s BIOS setup, another video card if you can located it, swap memory sticks, etc).
Also make sure your CPU is well ventilated, as it might (not likely, but I’ve seen it before) be getting thermally stressed. If all that doesn’t cure the reboots, I would then have to fault the mobo itself.
Hope this helps,